This invention includes embodiments that may relate to catalysts. This invention includes embodiments that may relate to methods of making catalysts. This invention includes embodiments that may relate to articles that include catalysts.
Exhaust gas streams may contain nitrogen oxides (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO). It may be sometimes desirable to control and/or reduce the amount of one or more of the exhaust gas stream constituents. NOx is thermodynamically unstable but does not spontaneously decompose in the absence of a catalyst. Exhaust gas streams may employ exhaust treatment devices including a catalyst to remove NOx from the exhaust gas stream.
Examples of exhaust treatment devices include: catalytic converters, evaporative emissions devices, scrubbing devices, particulate filters/traps, adsorbers/absorbers, and plasma reactors. Catalytic converters may include three-way catalysts, oxidation catalysts, selective catalytic reduction (SCR) catalysts, and the like. Scrubbing devices may remove hydrocarbon (HC), sulfur, and the like. Plasma reactors may include non-thermal plasma reactors and thermal plasma reactors.
Three way catalysts (TWC) deployed in catalytic converters may facilitate the reduction of NOx using CO and residual hydrocarbons. TWC may be effective over a specific operating range of both lean and rich fuel/air conditions and in a specific operating temperature range. This purification of the exhaust gas stream by the catalytic converter depends on the exhaust gas temperature. The catalytic converter works optimally at an elevated catalyst temperature, at or above about 300° C. The time period between when the exhaust emissions begin (i.e., “cold start”), until the time when the catalyst heats up to a light-off temperature, may be referred to as the light-off time. Light-off temperature is the catalyst temperature at which fifty percent (50 percent) of the emissions from the engine are being converted as they pass through the catalyst.
One method of heating the catalytic converter is to heat the catalyst by contact with high temperature exhaust gases from the engine. This heating, in conjunction with the exothermic nature of the oxidation reactions occurring at the catalyst, will bring the catalyst to light-off temperature. However, until the light-off temperature is reached, the exhaust gases pass through the catalytic converter relatively unchanged. In addition, the composition of the engine exhaust gas changes as the engine temperature increases from a cold start temperature to an operating temperature, and the TWC is designed to work best with the exhaust gas composition that is present at normal elevated engine operating temperatures.
Selective Catalytic Reduction (SCR) may use ammonia that is injected into the exhaust gas stream to react with NOx over a catalyst to form nitrogen and water. Three types of catalysts have been used, including base metal systems, noble metal systems and zeolite systems. The noble metal catalysts operate in a low temperature regime (240° C. to 270° C.), but are inhibited by the presence of SO2. The base metal catalysts, such as vanadium pentoxide and titanium dioxide, operate in the intermediate temperature range (310° C. to 400° C.), but at high temperatures they tend to promote oxidation of SO2 to SO3. The zeolites can withstand temperatures up to 600° C. and, when impregnated with a base metal, have an even wider range of operating temperatures.
Selective Catalytic Reduction with hydrocarbons reduces NOx emissions. Organic compounds can selectively reduce NOx over a catalyst under excess O2 conditions. However, the conversion efficiency was reduced outside the temperature range of 300° C. to 400° C.
It may be desirable to have catalysts that can effect NOx reduction across a wide range of temperatures and operating conditions. It may be desirable to have a catalyst that can effect NOx reduction using unmodified diesel or other hydrocarbon reductants that on the average have less than 4 carbon atoms per molecule. It may be desirable if the method and apparatus could be implemented on existing engines and did not use large inventories of chemicals.
Selective Catalytic Reducing agents for catalytic converters are used to reduce NOx emissions for lean burning engines. Lean burn engines, for example some diesel engines, operate at higher air to fuel ratios which can be as high as 65:1 (or as low as 12:1), as compared to conventional engine fuels that have air/fuel ratio of 14:7 for stoichiometric conditions. The composition of the air/fuel mixture is described by the air/fuel ratio of λ normalized to stoichiometric conditions. In the case of rich mode, rich air/fuel mixture or an air deficiency, the λ is less than 1; and, in the case of a lean burn mode, lean air/fuel mixture or an excess of air the λ is greater than 1.
A drawback to the lean burn engines is that they produce higher concentrations of NOx. Accordingly, catalytic converters are provided such as the SCR's. However, sulfur dioxide emitted by the lean burn engines is oxidized on the metal catalyst of the converter producing sulfur trioxides which in turn form sulfates in the presence of water (in the form of vapor) in the exhaust. These sulfates are thermally stable and compete with the storage of NOx at the converter. Accordingly, the capacity of the catalyst in storing nitrogen oxide is reduced.
It is known to regenerate the catalyst through desulfation processes in which the catalyst is subjected to reducing conditions under a “rich” air/fuel mixture. In addition, the temperature of the exhaust gas or stream from the engine is increased in order to heat the catalyst. Typically, the temperature of exhaust from an engine ranges from about 200° C. to 400° C. under typical operating conditions. Desulfation typically requires temperatures of at least 600° C. The engine may be operated between lean and rich modes/phases with the result that hydrogen (H2), carbon monoxide (CO) and unburned hydrocarbons (HC) are present in the exhaust gas as reducing agents. In this manner, sulfur species deposited on the catalyst are oxidized and expelled from the catalyst.